Didn't they directly say they changed gameplay elements? If so, which I believe is the case, a large part of the appeal for these remakes would be to see what has changed in the modernization process.
So, short answer, yes. Gameplay of a remake of a game promising updated gameplay seems pretty important.
They changed the control scheme but that is not something typically shown off in a gameplay trailer. You can also restart missions as soon as you fail them but that's self explanatory, easy to visualise and not something that requires a trailer.
The core game, such as going to a person, picking up a mission, buying clothes, buying property etc probably has not changed and is something you can just YouTube the older entries for. There are entire gameplay playthroughs out there for you to see.